During the operation of a machine, it may be desired to change the machine's direction of propulsion (i.e., forward or backward). When such a direction change is requested, a machine control system initially controls a power train to retard motion in the current direction. Typically, the retarding event is initiated by adjusting the transmission (e.g. downshifting) to drive power into the engine, thereby using parasitic losses to slow the machine down. Driving the power into the engine typically causes the engine speed to increase. If the engine is supplied with fuel during the retarding phase, the engine will generate power that can conflict with the power being dissipated. This may reduce the retarding capability of the engine. Therefore, during retarding events, the supply of fuel to the engine is reduced or cut off.
The retarding phase of the propulsion direction change event ends when the machine starts traveling in the opposite direction and the acceleration phase begins. During the acceleration phase, the transmission no longer directs power into the engine, and the parasitic losses cause the engine slow down. In addition, the control system places an acceleration load on the engine, which causes the engine to transfer to the transmission, any power absorbed during the retarding phase. This further contributes to the decrease in engine speed. To counter the decrease in engine speed and generate enough torque to meet the acceleration demand, the control system resumes or increases the supply of fuel to the engine. However, there is a delay between the resumption or increase of fuel and the generation of an adequate amount of torque to meet the acceleration demand. During this delay, the engine speed may continue to decline and may even fall below a minimum desired speed. Permitting the engine speed to fall below the desired minimum speed can adversely affect engine performance during a propulsion direction change event.
One method that has been employed to prevent the engine speed from falling below the minimum desired engine speed during a propulsion direction change event is disclosed in U.S. Pat. No. 4,769,774 (the '774 patent) issued to Narita et al. on Sep. 6, 1988. During a propulsion direction change event, the system disclosed in the '774 patent determines when the engine speed is accelerating and terminates fueling. As the propulsion direction change event is performed, the system continually monitors the vehicular speed and the engine speed. When the vehicle's ground speed approaches zero or the engine speed falls to a predetermined speed that is slightly greater than the engine's stall speed, the system resumes fueling. This allows the control system to resume fueling before the vehicle is propelled in the opposite direction.
Although the system disclosed in the '774 patent resumes fueling before any acceleration in the opposite direction occurs, improvements to engine performance may be limited. In particular, the control system utilizes only one predetermined threshold speed for triggering the resumption of fueling to prevent the engine speed from falling below a minimum desired speed. However, such a threshold may not be effective for all propulsion direction change events because the load placed on the engine during the acceleration phase varies from direction change event to direction change event. Increased loads may cause greater delays between the resumption of fueling and the generation of adequate torque to meet the load. This is because it may take longer to generate greater amounts of torque. In contrast smaller loads may cause minimal delays between the resumption of fueling and the generation of adequate torque to meet the load.
The disclosed system is directed to overcoming one or more of the problems set forth above.